Mechanical Impedance tuners are widely used in characterization of RF devices. The most common type of impedance tuner is the slab line tuner, which typically includes a 50 Ohm TEM slab line, with a movable mismatch probe. FIGS. 1A and 1B illustrate such a tuner in simplified form, with a center conductor 10 supported between opposed conductive ground planes 12, 14 which form the slab line. A mismatch probe 20 is supported for movement along or horizontal to the center conductor, and also in a direction traverse to or vertical to the center conductor. If the mismatch probe is moved out of the electrical fields of the slab line, it has almost no effect, allowing the slab line to look like a good 50 Ohm line. However, if the mismatch probe is moved close to the center conductor, the electrical fields are affected, causing a mismatch. The magnitude of the mismatch is controlled primarily by adjusting the distance of the probe from the center conductor. The phase of the mismatch is controlled by moving the probe in a direction parallel to the center conductor. A similar effect can also be done in other transmission line environments, such as waveguide.
Automated impedance tuners typically use stepper motors to provide remote computer control, to move the probe vertically relative to the center conductor and to move a carriage holding the probe horizontally along the center conductor. The stepper motors are controlled by sending current pulses to the motor, and each current pulse will move the motor by a known amount based on the physical construction of the motor. This means that the control electronics can keep track of the tuner position without any direct position feedback, as long as power is continuously applied.
However, when power is first turned on, position of an impedance tuner with stepper motors is not known. A process called initialization is then performed to determine the position and set the zero point. The initialization consists of moving a motor in one direction until a limit sensor is encountered. The position at or near the limit is then set to zero, and all subsequent moves are relative to that zero position.
Some tuners with multiple carriages do not have a wall between the carriages, allowing the travel range of the carriages to overlap. In this case, the first carriage is initialized at a fixed limit at one end of the tuner. The second carriage is initialized relative to the first carriage while the first carriage is in a known position. This relative initialization is therefore variable and frequency dependent. If there is a third carriage, it would be initialized relative to the second carriage while the second carriage is in a known position. One disadvantage of this method is that initialization of the second carriage cannot begin until the initialization of the first carriage is complete.